1. Field of the Invention
This invention relates to photopolymerizable compositions, to polymerized products obtained therefrom, and to processes for the photopolymerization of ethylenically unsaturated compounds. More particularly, this invention relates to the discovery of a novel class of photosensitizing compositions which may be used to promote the photopolymerization of ethylenically unsaturated compounds in the presence of visible light.
2. Brief Description of the Prior Art
The polymerization of ethylenically unsaturated monomers with various compounds, such as peroxides and metal carbonyls, is known. See Bamford et al., Proceedings of Chemical Society (1962), page 110, CA 57, 12690y, who disclose the polymerization of methyl methacrylate at 100.degree. C. with a metal carbonyl, such as iron carbonyl, dissolved in a halogen compound, such as carbon tetrachloride and chloroform.
The photopolymerization of ethylenically unsaturated organic compounds by exposure to high intensity radiation, such as, ultraviolet rays, is also known in the prior art. It is also known that photopolymerization initiators, often referred to in the art as photosensitizers or photoinitiators, may be added to the ethylenically unsaturated organic compounds to accelerate their polymerization rate on exposure to high intensity radiation. See Strohmeir et al., Naturforsch 19b 882 (1964), CA 62, 2825e; U.S. Pat. Nos. 2,850,445 and 2,875,047; McCloskey et al., Industrial and Engineering Chemistry, October, 1955, pages 2125-2129; and Bamford et al., Proc. Roy. Soc. (London), A 284, 455 (1965). These photopolymerization initiators absorb light rays and, as a result, free radicals are formed which are capable of initiating polymerization. Although the photopolymerization initiators described heretofore do accelerate the polymerization of ethylenically unsaturated compounds on exposure to high intensity radiation, the practical applications thereof have proven to be somewhat limited as the photopolymerization initiators proposed heretofore suffer one or more disadvantages. A major disadvantage of numerous photopolymerization initiators proposed in the prior art is that they require the use of ultraviolet radiation or at least the use of a light source which contains a substantial component thereof of ultraviolet radiation. Another disadvantage of many photopolymerization initiators proposed heretofore is that they require the polymerization to take place in the absence of air since the oxygen in the air tends to inhibit photopolymerization, especially when the material to be polymerized is in the form of a thin film. Various methods of excluding oxygen have been proposed, but they have proven in practice to be too cumbersome and expensive to use. Accordingly, there is a great need in the art for an improved photopolymerization initiator which will function in the presence or absence of atmospheric oxygen and which does not require the presence of ultraviolet radiation but which will sensitize the photopolymerization of ethylenically unsaturated compounds on exposure to visible light rays having a wave length from 4,000 to 7,000 angstroms.